Halachically "משנכנס אדר מרבין בשמחה" is not acceptable
(according to the Rambam or the Shulchan Aruch)
The Gemoro (Ta’anis 29a ) "In the name of Rav it was said,
the same way, when the month of Av comes in, you reduce
with Simcha, so too when the month of Adar comes in you
add with Simcha".
The Rambam and Sh. Aruch bring the Halacha of
M'shenichnas Av Mema'atin B'simcha.
They do not* mention M'shenichnas Adar Marbin B'simcha.
The obvious reason is, there is no such Halacha to be
Marbeh B’simcha when Adar comes in.
The Chasam Sofer (O.Ch 160) explains
why we don't Pasken Marbin Besimcha in Adar.
* The Magen Avraham does mention it
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